Filtration is typically performed to separate, clarify, modify and/or concentrate a fluid solution, mixture or suspension. In the biotechnology pharmaceutical and medical industries, filtration is vital for the successful production, processing and analysis of drugs, diagnostics, chemicals as well as many other products. As examples, filtration may be used to sterilize fluids or gases, clarify a complex suspension into a filtered “clear” fraction and an unfiltered fraction; similarly, constituents in a suspension may be concentrated by removing or “filtering out” the suspending medium. Further, with appropriate selection of filter material, filter pore size or other filter variables, many other specialized filter uses have been developed; these may involve selective isolation of constituents from various sources, including, cultures of microorganisms, blood, as well as other fluids that may be solutions mixtures or suspensions. With further advancements in cell and recombinant DNA technologies many new products are being developed, many of which are so complex that they can only be produced by the complex synthetic machinery of live cells, using cell culture techniques. Filtration may be used to enhance the productivity of such cell cultures by maintaining the cultures for extended periods at high cell concentrations at high productivity and by providing a product stream more amenable to further processing and purification.
Filter chemistries, configurations and modalities of use have been developed to facilitate separation of materials according to their chemical and physical properties; In spite the extensive developments in filter technology, they are generally limited by their tendency to clog; for example, when used to filter a suspension of cultured mammalian cells they tend to clog with dead cells, cell debris, aggregates, fibrous biomolecules or other constituents found in the complex “soup” of a culture. In this regard, the method of filtration can have a profound effect on the filtration efficiency and the longevity of the membrane. In one kind of filtration process, commonly known as “dead end” filtration, the entire fluid is passed through the membrane perpendicular to the membrane surface. Debris rapidly accumulates at the surface resulting in rapid blockage of the membrane. Typically, application using dead end filtration involves small samples. The process is simple and relatively inexpensive. Another filtration process, generally known as Tangential Flow Filtration (also known as TFF) offers an improvement over dead end filtration. In TFF, fluid to be filtered is recirculated with a pump, typically, from a reservoir through a filter and back to the reservoir. The flow through the filter is parallel to the surface of the filter. Any accumulation of debris is effectively removed by the “washout” effect of the circulating fluid; nevertheless, one of its limitations is the tendency to form a gelatinous deposit on the filter surface, which may limit the effectiveness of the filter and eventually clogging it. Another processes, known as alternating tangential flow filtration, offers yet another mode of filtration; It is similar to TFF, in that it generates a flow pattern parallel to the filtration membrane surface; however, it differs from TFF in that the direction of flow is repeatedly alternating or reversing across the filter surface. If a change in flow direction is described using a complex pathway of tubing, valves and pumps, placement of such components in a culture flow path adds sheer to the system and provides sites for cell to aggregate and potential clogging sites, nor are such systems very amenable for sustaining homogeneous cultures. The alternating tangential flow filtration system described in U.S. Pat. No. 6,544,424 consists of a filter element, commonly a hollow fiber cartridge, connected at one end to a reservoir containing the content to be filtered and at the other end the filter its connected to a diaphragm pump capable of receiving and reversibly expelling the unfiltered liquid flowing reversibly between reservoir and pump through the filter element. The system has shown the ability to sustain filtration of a complex mixtures, Including the medium of a cell culture, even when that medium is burdened with high cell concentration and other cellular products. That system, however, is limited in its range of applications.
The use of animal cell culture is increasingly used for production of various cell derived biologicals that may be natural or engineered, including proteins, nucleotides, metabolites and many others. Accordingly, methods of production may also vary. They may range from the use of “simple” batch to continuous processes. In a normal batch culture production processes, cells are first inoculated into a fresh medium, after which the cells rapidly enter a logarithmic growth phase. As they consume the nutrients in the medium, waste products accumulate; concomitantly, cells transition from rapid growth to a stationary growth phase followed by a cellular decay phase. While several methods have been developed to optimize batch culture production, in each case, these processes undergo rapid growth and decay cycles. Another culture process involves maintenance of the culture continuously using a process commonly known as perfusion. In a perfusion culture, waste products generated by the cells are continuously removed from the culture, while retaining the cells. Removed waste medium is replenished with fresh medium. With this method, it is possible, therefore, to achieve a state of equilibrium in which cell concentration and productivity are maintained. Typically, about one to two culture volumes are exchanged per day and the cell concentration achieved in perfusion is typically 2 to more than 10 times that achieved at the peak of batch culture. Yet, in spite of the great benefits of the perfusion process, its acceptance has been slow. One reason for this slow acceptance may be inherent in the fact that most products originate at small scale in a batch culture system, like a “T” flask. If more material is needed, it is generally produced by increasing the number and size of the “T” flasks or transferring the culture to roller bottles or spinner flasks, both of which are typically also batch cultures by nature. By the time the culture is scaled to a bioreactor, the process has been largely biased by the previous handling of the culture. It would therefore be desirable to create a disposable perfusion system that is more accessible at small scale, at the level of research and development. Attempts to address this issue with hollow fiber bioreactor or other solid bed bioreactor, in which the cells grow attached or entrapped to a fixed surface, are only partially effective; their inherent inhomogeneity and inaccessibility to the cells limits their usefulness as a research tool. It would desirable to create a system that is easily scaled down, that would maintain cultures homogeneously in continuous perfusion, such that sampling, modifying or monitoring any part of the culture will reflect the conditions in the entire culture. An investigator may tap into such a continuous culture as the need arises for cells, for analysis or for a desired product to study the behavior of the cells in a continuous steady state culture. An investigator can make essential modifications to such a culture followed by observing the cultures response. In addition to proving a means for generating product, such a continuous culture may offer a powerful research and development tool. The proposed invention addresses this issue by providing a perfusion bioreactor system, that maintains a homogenous culture which is accessible to manipulation, sampling and analysis. The proposed system may be provided in a convenient sterile form ready for use and readily disposable.
With advancements in new materials, manufacturing methods and requirements in recent years, the construction and use of disposable equipment has gained increasing acceptance. The use of disposable bags as cell culture bioreactors and storage vessels has become more common. Such disposable containers can be “set-up” with minimal handling and do not require cleaning or sterilization by the user. They are supplied clean, sterile and in a form ready for use, at great savings in cost and reduced handling by the user; furthermore, at the end of their use, the bags can be readily discarded without disassembly or cleaning. The disadvantage of the bags lies in their inherent fragile nature, limiting their size; although, significant progress has been made in the construction of large disposable bags. Another disadvantage of the bags is the limited ability to agitate or mix the culture. Linear scale up of mixing is difficult to sustain with increasing bag size. While the bag volume increases by the cube, the surface area of the culture head space increases by the square; oxygen transfer becomes limiting as is growth and cell productivity. There are also limitations on monitoring the conditions of the culture with pH, oxygen or other probes, factors which can profoundly effect the reproducibility of the culture and limit its achievable cell concentration and productivity. Considerable progress has been made by some bag manufacturers to solve the problem of agitation by incorporating an impeller into the bag, additionally, means for sampling, monitoring and making changes to the culture within a bag are being developed; in spite these developments, however, these bags are used to grow cells in batch or fed batch.
It would be, therefore, desirable to incorporate a device that would enhance the productivity of disposable bags or similar systems and alleviate some of their shortcomings. Some desirable features of such a device may incorporate the following features, including: (i), the ability to facilitate mixing of the culture within the disposable bag, (ii), the ability to retain cells and sustain the culture in continuous perfusion mode, (iii), include the capacity to be used externally so that it may be replaced in mid process with minimum disruption to the process, including maintaining process sterility, (iv), remain fully or partially disposable in nature. One can envision the alternating tangential flow filtration system described in U.S. Pat. No. 6,544,424 as encompassing the above requirements as well as a disposable system, since in its description the device was not limited to construction material nor to its methods of assembly; however, the system in U.S. Pat. No. 6,544,424 does not describe a system that may be used as a complete culture system, eg, a system that incorporates or combines the culture vessel and the perfusion device into a single apparatus. The current invention, as will be described, may be used as such an apparatus that, in addition to providing a means for continuous culture, may be fully disposable, and also offers other benefits and uses.
Changing pressures gradients and flows that are both parallel (axial flow) and perpendicular (transmembrane flow) to the membrane surface are inherent in the alternating tangential flow process. During the “pressure cycle”, the pressure in the pump is greater than the pressure in the retentate reservoir. The retentate flows “forward” from the diaphragm pump, i.e., through the filter element towards the retentate reservoir. Also, some of the liquid is forced across the filter membrane into the filtrate compartment. Therefore, with an enclosed filtrate compartment, the influx of filtrate can pressurize the filtrate compartment. Conversely, during the “exhaust, cycle” of the alternating tangential flow filtration process, the pressure in the pump is less than that in the retentate reservoir, so that liquid flows in reverse, from reservoir to pump. Additionally, during the exhaust cycle, filtrate compartment fluid pressurized during the previous pressure cycle will also flow in reverse, from the filtrate compartment to the retentate compartment. The backflow produces a back flush component that maintains the membrane and inhibits clogging. This effect is further enhanced by another kind of transmembrane flow, one which forms when the resistance to axial flow inside the hollow fiber, or lumen side, is greater than in the external, shell, side of the hollow fiber. Therefore, during the pressure cycle pressurized fluid forced into the inlets of the hollow fibers will take the path of least resistance, or in proportion to the resistance, and the fluid will flow not only through the lumens, but also in part across the membrane, into the filtrate or shell side, as previously described. An axial pressure gradient forms on both sides of the filter causing fluid flow towards the exit end of the filter. Towards the exit end of the hollow fiber, the lumen pressure decreases towards its minimum. The pressure drop inside the lumen relative to the shell side results in filtrate reentry into the lumen or retentate stream, on its way to the retentate reservoir. The flow from the filtrate side back into retentate side provides additional back flushing. It is obvious that such a flow will also be observed during the exhaust cycle, but in the reverse direction. The process thus provides back flushing capacity at both ends of the filter element. As a result, it is obvious that the described flows offer a great capacity for exchange of fluids between the retentate and filtrate sides. Such exchange can be highly beneficial for processing fluids. It is also the objective of this invention to use this capacity of the alternating tangential flow filtration system for exchanging fluids across a membrane by modifying the configuration of that system in a manner that would result in unique systems and provide great improvement over existing devices, beyond its previous use as strictly a filtration device.
As will be shown, by specific compartmentalization of the alternating tangential flow filtration process, one can produce systems that may greatly improve processing of blood, convert the system into a disposable perfusion bioreactor, facilitate certain biological and chemical reactions or be used for purification or isolation of certain constituents from biological or other fluids. Other processes and uses are also possible as will become apparent.